The decade of the '90s could confront America with a water shortage as serious as the current oil-price inflation, a top government water expert warns.

"As we move toward the 21st century, short supplies of clean water could rival expensive oil as one of the nation's most serious concerns," Michael D. Hudlow said Tuesday."I believe we're headed into a water management crisis in the 1990s," said Hudlow, director of the National Oceanic and Atmospheric Administration's Office of Hydrology.

The severe drought that struck much of the nation in 1988 was just the first round in what could become a continuing problem, Hudlow said.

"We're going to have real extreme drought conditions going into next spring . . . unless Mother Nature cooperates and gives us a bumper crop of snowfall in the mountains of the West," he said.

In general, the water outlook in much of the West is precarious, depending heavily on the winter snows, he said at a news conference called to discuss water conditions and resources.

"Extreme drought still grips much of the West, from California to Montana, and in parts of the Southeast. In mid-October, drought conditions in much of Georgia and the Carolinas ended with a significant flooding episode," Hudlow reported.

Water shortages are developing as part of the normal variability of climate, he said, with the relatively wet years of the 1970s and early 1980s giving way to a drier period.

And that does not include the threat of the so-called greenhouse effect, which some scientists say could raise temperatures worldwide and change rainfall patterns.

While forecasting floods and drought now depends on climate records and short-term weather forecasts, Hudlow said a new water-supply forecasting system is being developed by his agency.

The Water Resources Forecasting Service, called WARFS, is being tested at the Colorado River Basin Forecast Office in Salt Lake City.

Using long-range climate and weather models, the system can study the conditions in a local river basin and calculate probabilities of floods, drought or a return to normal conditions seasonally and farther into the future.

If the testing goes well and money is available, the system could be in national service by 1992.

It could provide local water managers and civil defense officials the probabilities of danger or drought on which they could make decisions, Hudlow said.

For example, reservoir managers might be reluctant to reduce water levels for fear they might later have too little water for generating power, irrigating crops and other uses. But if they had a reliable forecast of a good chance of severe flooding, they could decide to lower reservoir levels to trap flood water and prevent damage.